1. Field of the Invention
The present invention relates to a tandem laser scanning unit (LSU). More particularly, the present invention relates to a tandem LSU capable of realizing a multi-color image by simultaneously scanning light beams emitted from a plurality of light sources onto different photosensitive drums.
2. Description of the Related Art
A laser scanning unit (LSU) for apparatuses such as laser printers, digital copying machines, barcode readers, and facsimiles forms a latent image on an exposure surface using a main scanning operation performed by a beam deflector and a sub-scanning operation performed by rotating a photosensitive drum. In particular, to realize a multi-color image, a tandem image forming apparatus includes a plurality of exposure bodies that correspond to corresponding color components. An example of a tandem image forming apparatus is a color laser printer. A tandem image forming apparatus uses a tandem LSU simultaneously forming monochromatic light that corresponds to respective color components (for example, yellow, magenta, cyan, and black) on corresponding photosensitive drums, respectively.
The tandem LSU includes a beam deflector disposed on a light-emitting path of a light source and that deflects light beams. When the beam deflector is shared with respect to a plurality of light beams, the number of optical elements and the structure thereof may be simplified, so that an image forming apparatus including an LSU may be compactly made and the manufacturing cost thereof may be reduced. Considering such advantages, tandem LSUs having a structure in which a beam deflector is shared with respect to a plurality of light beams are actively being developed. To independently scan the plurality of light beams on a plurality of corresponding photosensitive drums, it is required to direct the plurality of light beams to a beam deflector rotating at high speed and to spacially separate the light beams after the light beams are deflected. For that purpose, different monochromatic light is incident to a rotating beam deflector in an oblique direction, so that the monochromatic light may be spacially separated (Japanese Patent Publication No. 2004-361627). However, in such an oblique incidence optical system, a straight scanning line is not formed on a photosensitive drum and the scanning line is distorted in a curved shape, such as a bow. Such distortion of the scanning line deteriorates image quality because the pitch between the scanning lines is not maintained uniformly. Particularly, in a color image forming apparatus realizing a color image by overlapping different monochromatic images, monochromatic images on a transfer belt do not coincide with each other, so that a desired color and image may not be formed.
Accordingly, a need exists for an improved tandem laser scanning unit to substantially eliminate distortion of a scanning line.